The enzymes for the synthesis of isoleucine and valine in Neurospora are located in the mitochondria. Attempts are being made to determine how this system is integrated into the mitochondria, and whether it exists as a complex. Mitochondrial nucleases and the transcription and translation processes are being studied in Neurospora mitochondria. The relation between mitochondrial DNA and respiration and oxidative phosphorylation in Neurospora and mouse mitochondria will be studied.